OutgoingAuthorities event is triggered too many times.

We have to choose between a single OutgoingAuthorities event with a list or as many OutgoingAuthority event as outgoing authorities. Here we have a mix:

        for member_id in members_ids_to_del.iter() {
            T::OnOutgoingMember::on_outgoing_member(*member_id);
            Self::deposit_event(Event::OutgoingAuthorities {
                members: members_ids_to_del.clone(),
            });
        }

Discovered in https://forum.duniter.org/t/number-of-blocks-in-a-babe-session/12842/3.